1. Field of the Invention
This invention relates to delayed coking, and more particularly to a method of increasing the capacity of a delayed coker unit by reducing the cycle time of the unit.
In a typical delayed coker unit, a pair of coke drums are alternately filled and emptied, with coker feed being pumped into one of the drums while the other drum is being emptied of coke and prepared for the next filling cycle. The capacity of a delayed coker is determined by several factors including the size of the coke drums, furnace capacity, pumping capacity, and the cycle time. As the drum size, furnace and pumping capacity are not easily changed, sometimes the only feasible way to increase coker capacity is to reduce the cycle time, thereby allowing more drum fills in a given time period.
2. Background Art
A conventional coking operation includes, in the process of emptying the filled drum, the steps of steaming out the filled drum to remove residual volatile material from the drum, quenching the steamed out coke bed with water, draining quench water from the drum, opening the top and bottom of the coke drum (unheading the drum), drilling a pilot hole in the coke bed from the top, drilling out the remaining coke with a radially directed jet drill, removing the drilled out coke from the bottom of the drum, closing the top and bottom openings of the coke drum, and preheating the empty coke drum by passing hot vapors from the other drum being filled with hot coker feed. The preheating step is necessary to bring the empty coke drum temperature up prior to switching the hot coker feed to the recently emptied drum, as otherwise the thermal stresses from feeding hot feed into a relatively cool drum would cause serious damage.
When capacity is not a problem, the preheat step can take place over a considerable time period, and the thermal stresses are manageable. When capacity becomes an issue, one way to increase it is by reducing cycle time, enabling production of more drums of coke in a given time period.
The preheat step discussed above is a significant part of the cycle time, and is the area that holds the most potential for cycle time reduction, as many of the other steps in the cycle are more or less fixed, or in any event not easily reduced without significant capital requirements.
A typical coke drum is supported by a skirt which is welded to the drum near the junction of the drum shell and the lower cone of the drum. The maximum thermal stresses occur at the time the hot oil feed, at about 900.degree. F., is switched to the preheated drum. These thermal stresses are partly due to the fact that the interior surface of the preheated drum is hotter than the exterior of the drum, including the area where the supporting skirt is welded to the drum shell. The expansion rate of the interior of the shell, upon being contacted with hot oil feed, is initially greater than the expansion rate of the cooler exterior portion. If sufficient time is available, the preheat step can be carried out over a time period sufficient to heat the drum exterior to a temperature near that of the drum interior. However, this is a problem if preheat time is to be minimized in order to reduce the overall cycle time. There has been a continuing need for a method of reducing cycle time without exacerbating the thermal stresses in the drum, particularly in the area near the junction of the drum and its supporting skirt.